1. Field of the Invention
This invention relates generally to disposable wet-packaged skin cleansing fabrics or cloths formed of paper or other nonwoven fibrous webs of the kinds generally known in the art as towelettes, wet-wipes, fem-wipes, and the like. It is particularly concerned with the provision of such fabrics which will retain suitable wet tensile strength during storage and use but which can be readily disposed of by flushing in water without danger of clogging the plumbing system.
2. Prior Art
Wet-packaged skin cleansing and refreshing tissues are well-known commercially, generally referred to as towelettes, wet-wipes, fem-wipes, and the like. Typical examples of such products are described in U.S. Pat. Nos. 3,057,467; 3,563,371; and 3,398,826. These may comprise an absorbent sheet made of paper, prepared or treated to impart wet strength thereto, having the dimensions of the usual wash cloth and packaged wet in folded condition individually in impervious envelopes or in multiples in closed containers. The liquid employed in pre-moistening the sheet is generally an aqueous alcoholic solution which may further contain a surface active detergent and a humectant and in some instances also a scenting agent. Instead of individual packaging of such moist sheets, these are often marketed in recloseable containers having any desired convenient number of such folded sheets. A typical example of such products particularly designed for use in feminine hygiene, popularly known as "fem-wipes", is disclosed in U.S. Pat. No. 2,999,265.
Certain of the earlier known products suffer from the drawbacks of loss of wet strength on account of being kept moist for even relatively short periods of storage, thereby interfering with their intended use by the consumer. Others of these known products which retain adequate wet strength, cannot be readily disposed of by flushing in water in conventional toilet bowls, since the binders employed in imparting wet strength do not disintegrate sufficiently and thus after cause clogging of the plumbing. In some instances, it has been advocated that acidic or alkaline materials respectively be added to the water empoyed in flushing the used cloths to assist in disintegrating the binder therein, these being selected in accordance with the nature of the resinous binder employed.
Polyvinyl alcohols (PVOH) are well-known in commerce for use in textile and paper sizing and coating, as adhesives, binding agents, dispersing/stabilizing agents for emulsions, and the like. These alcohols are generally manufactured by polymerizing vinyl acetate and hydrolyzing the acetate to an alcohol. The marketed grades of polyvinyl alcohol vary in degree of polymerization and degree of hydrolysis. As used herein, "fully hydrolyzed" products are those in which the vinyl acetate has been hydrolyzed to about 95% or higher and up to about 99% by weight. Polyvinyl alcohols having a degree of hydrolysis above 99% are designated as "super hydrolyzed". Films produced from polyvinyl alcohol grades having a degree of hydrolysis above about 95% are resistant to attack by cold water; the extent of water resistance increasing directly with increase in the degree of hydrolysis. Polyvinyl alcohols of lower degree of hydrolysis than the so-called fully hydrolyzed products, such as the "partially hydrolyzed" grades (80-95% by weight hydrolyzed) are almost completely soluble in water at room temperature, while the fully hydrolyzed products have more limited cold water solubility.
Changes in the degree of polymerization primarily affect solution viscosity; i.e. the viscosity of "fully hydrolyzed" and "partially hydrolyzed" products of low viscosity (in 4% aqueous solution at 20.degree. C.) are about 10 cps, medium viscosity are in the range of about 20-35 cps and high viscosity are in the range of about 40 cps and above. The viscosity of the aqueous solution of the polyvinyl alcohol is thus an indication of the degree of polymerization.
Also available on the commercial market are the socalled "tackified" polyvinyl alcohols. These are produced by controlled boration of polyvinyl alcohol as disclosed, for example, in U.S. Pat. No. 3,135,648. According to the patent, the wet tack of polyvinyl alcohol adhesives is increased by addition thereto of about 4 to 15% of a water soluble boron compound and an amount of acid such that the pH of the adhesive is below 5.5. The desired high wet tack adhesives are prepared by cooking the mixture of polyvinyl alcohol and borate compound in water.
Surface sizing of paper with aqueous mixtures of polyvinyl alcohol and boric acid is disclosed in U.S. Pat. No. 3,438,808. The boric acid in admixture in the amount of 15% or more of the polyvinyl alcohol and applied in heated condition to the web, inhibits the extent of penetration or migration of the sizing composition into the paper.
Boron compounds such as boric acid and alkali metal borates, are known to react with polyvinyl alcohol. Whereas boric acid is believed to react with PVOH to form a reaction complex having monodiol type bond, which complex is not a gel; by reaction of PVOH with borax (or other alkali metal borate) complexes having didiol cross-linkages are formed which are of gel nature. It was concluded by R. F. Nickerson (J.A.P.P. Polymer Science: 15, 111; 1971) that the borate ion is the effective cross-linking agent and that boric acid at concentrations greater than 0.03 M contributes borate ions in sufficient amount to provide a gel type complex. Thus, the reaction mechanism in the case of borax and boric acid respectively may be represented by the equations below: ##STR1##
Coating and sizing composition containing polyvinyl alcohol and a borate or boric acid are known in various arts. Thus, U.S. Pat. No. 2,324,601 discloses sizing compositions for synthetic linear polyamide knitting yarns comprising water soluble polyvinyl alcohol and boric acid, said size being removable by hot water washing of the yarn. It is also known to post-treat polyvinyl alcohol spun fibers following coagulation with boric acid solution, as well as the incorporation of limited amounts of boric acid or bromates into the spinning solutions to improve hot water resistance, or other high temperatures properties of the formed PVOH fibers (U.S. Pat. Nos. 3,170,973 and 3,850,901).
It is also known to employ certain resins and other polymeric materials as binders or coatings or nonwoven fabrics used as toilet-flushable disposable products such as wrappers or outside coverings for diapers and sanitary napkins, surgical dressings and the like, wherein such fabrics need have during their intended use sufficient tensile strength not to disintegrate while in contact with body fluid discharges. Among binders suggested for use in such fabrics are aqueous dispersions of mixtures of acrylic resins and polyvinyl alcohol, as disclosed for example, in U.S. Pat. No. 3,561,447. The use of cold water soluble polyvinyl alcohol in the absence of other resins or polymers as such bonding agent for disposable nonwoven fabrics, is disclosed in U.S. Pat. No. 3,654,928. To prevent premature structural weakening or disintegration of the fabric as a result of softening or dissolution of the polyvinyl alcohol binder in the presence of body discharge fluids; the polyvinyl alcohol film is oversprayed with a gelling or insolubilizing agent such as borax, which is stated to react with the polyvinyl alcohol and cross-link at least the exposed surface areas to a sufficient degree to render the reacted binder, when dried, somewhat water resistant. When the treated fabric is exposed to a large excess of water, the borax is said to be cross-linkaged in the polymer to reduce water resistance to a non-effective level.
In a subsequent patent by the same inventor, U.S. Pat. No. 3,689,314, certain of the shortcomings and disadvantages of the borax treatment proposed in the earlier patent are set forth. The later patent advocates application to the fabric reactants which form borax in situ only on heating, to overcome the problem of premature gelation of the polyvinyl alcohol solution by direct addition of borax thereto. Thus, in accordance with this patent, the web is treated with a cold water solution of polyvinyl alcohol containing boric acid and sodium bicarbonate and the web dried at elevated temperature to effect the desired reaction. Further modifications set out in U.S. Pat. Nos. 3,692,725 and 3,808,165. These patents respectively disclose addition of carbon dioxide or an unstable organic acid releasing carbon dioxide, into the polyvinyl alcohol-borate binder composition, to overcome asserted previous shortcomings.
Since borax disassociates in water to give borate ion, it is considered a more sensitive PVOH gelling agent than boric acid. In the initial experimental work leading to the present invention, it was surprisingly found that whereas a film of partially hydrolyzed PVOH (low viscosity 87-89% hydrolyzed) which can be dissolved in water within one minute, also dissolved in a 5% borax solution, but did not dissolve in a 5% boric acid solution. This discovery led to further investigation resulting in the present invention.